Extracellular polymeric substances (EPS) are a major fouling component in membrane bioreactor (MBR) systems. For a better understanding of the fouling mechanisms of EPS, the evolution of fouling of sodium alginate, a microbial polysaccharide was studied during crossflow ultrafiltration and microfiltration. Incremental flux-stepping experiments and long-term subcritical flux filtration were carried out. A two-stage of transmembrane pressure (TMP) profile reported during long-term filtration of MBR effluents was also observed for alginate solutions. An initial slow and gradual TMP increase was followed by a sudden transition to a rapid TMP rise. Alginate transmission and deposition pattern supported the concept of flux redistribution among open pores as a cause of two steps of fouling profile. A desorption method with sodium hypochlorite was developed to characterise the temporal and spatial distribution of alginate along the membrane surface. The TMP rise was increased, not only due to the increase of alginate loading on the membrane (from 52 to 252 mg m(-2) in days I and 6, respectively), but also because of the temporal increase of the specific cake resistance (from 5.9 to 16.1 x 10(15) m kg(-1) within 6 days). The fouling layer formed in the long-term subcritical flux operation appeared irreversible, while fouling layers formed in the short-term dead-end constant pressure or flux-stepping experiment showed greater reversibility. Membrane autopsy using FESEM technique also confirmed the mostly irreversible nature of the fouling layer during the long-term subcritical flux operation. (c) 2005 Elsevier B.V. All rights reserved.